1. Field of the Invention
The present invention relates to an image forming apparatus having a fixing device for fixing an image which includes a pair of rollers and a heater is housed in at least one of the rollers.
2. Description of the Prior Art
Conventionally the so called heat roller system is widely used as a fixing device for an imaging apparatus such as an electro-photographic printer or copy machine. In this system, a heater is installed as a heat source in at least one of two rollers. A toner image is fixed thermally when a copy paper passes between the two rollers. The heater is automatically controlled so as to maintain a temperature or a range of temperatures of the roller desirable for fixing a toner image.
In order to control the heater, there are usually provided a thermister for detecting a temperature of the surface of the heat roller, a bridge circuit such as a Wheatstone bridge circuit including the thermister as an element thereof and a comparator for comparing an output of the bridge circuit with a predetermined value or a microcomputer for processing an output of the bridge circuit (See, for example, JP-A 53-65743 and JP-A 60-115977).
However, in this temperature control, the heater may improperly turn on if a power supply circuit is damaged or the temperature controller becomes uncontrollable. Once the heater is improperly turned on, the heat-roller is overheated so as to cause thermal damage of the heat-roller or parts arranged therearound and, furthermore, a fire may occur due to a burning of a copy paper inserted between the pair of rollers.
Conventionally, a thermo-sensitive braker such as a thermal fuse is arranged near the surface of the heat-roller and when the heat roller is overheated, the power supply to the heater is automatically cut off. However, the sensitivity of the thermo-sensitive braker is relatively low in general and, therefore, it is difficult to prevent thermal damage of the heat-roller since the operation temperature thereof is set at a temperature which is higher than the upper limit of the desirable temperature range by a proper temperature.
In order to solve the problem mentioned above a means for preventing the improper turning on of the heater has been proposed conventionally. However, according to the conventional system, the thermister for detecting the surface temperature of the heat roller is used as means for detecting the improper turning-on of the heater and it is presumed a priori that the thermister always operates normally (if the system is controlled by a control program, the control program is presumed to be performing normally). Accordingly, the protection system is inoperative in the case that the thermister is defective and/or the control program is not performing normally. As causes which invite the inoperability of the thermister, adhesion of toner and/or paper dust to the surface of the thermister and fricative waste thereof by the surface of the heat roller are considered. Furthermore, the microcomputer for controlling the system may become uncontrollable due to noise caused when individual imaging units are actuated and/or a lighting surge. As is apparent from the above, these usually happen during a copying operation. In other words, the possibility of overheating of the heat-roller during the copying operation is considered to be higher than that in the waiting state of the machine.